Repair Rejected without reason

I sent my under warranty iphone for repair at MTN BOARDWALK. After 3 weeks store contacts me that my phone has arrived from the MTN repair centre. I Get to the store and find out that my phone unrepaired because it was rejected by HVRC. I contacted HVRC and was informed that there was a serial mismatch With Phone, they said my phone was booked with incorrect documents And That The Phone Didn't Even Arrive At The HVRC hence passing buck onto MTN BOARDWALK For causing the error. Upon calling MTN BOARDWALK to ask if they were at fault here. They say no, because, if there was a real serial mismatch RTT(courier) would have picked it up in store and the phone would be rejected right there in the shop. MTN BOARDWALK does have a delivery waybill attached to the parcel that confirms On the 21/01 RTT collected, on the 28/01 they were still awaiting status on device and on the 03/02 RTT delivered the device back to MTN BOARDWALK. As a MTN customer I am dissatisfied at the events leading up to me writing this complaint. And the way MTN HVRC passed blame onto MTN BOARDWALK For supplying incorrect documents for device. I just want my iPhone fixed! Unhappy customer, 3 weeks of time wasted!